1. Field of the Invention
The present invention relates to a vehicular occupant protection system such as an air bag system and a pre-tensioner for a seat belt provided in a vehicle and a crash mode determining unit and a crash mode determining method therefor.
2. Discussion
An acceleration sensor 1 of a vehicular air bag system is disposed for example in the vicinity of a console box on the floor of the cabin of a car as shown in FIG. 11 to detect acceleration of the vehicle. The system determines whether or not a crash has occurred by comparing the detected acceleration with a single threshold value. When the system determines that a crash has occurred, it spreads an air bag by turning on an activation unit to protect an occupant of the car from the impact of the crash.
However, because the above-mentioned air bag system merely determines whether or not the crash has occurred by comparing the detected acceleration with the single threshold value, it is unable to discriminate even to a mode of the crash.
For instance, it is unable to discriminate a head-on crash against a car running in the opposite direction while running at low speed from a crash against a pole while running at intermediate speed. As a result, it has had a problem that its determination timing is late in a case of the crash against the pole.
Examining the cause of the above-mentioned problem in detail, it was found that an integrated value of the detected acceleration (hereinafter referred to as a velocity change) changes as time elapses as shown by a curve L1 in FIG. 12 in a case of the head-on crash. Meanwhile, the velocity change changes as time elapses as shown by a curve L2 in FIG. 12 in a case of the pole crash.
Therefore, when it is determined whether or not the crash has occurred using the single threshold value in the respective cases of the head-on crash and the pole crash, the determination timing of the pole crash is late as compared to the determination timing of the head-on crash as it is apparent by comparing the states of changes of the both curves L1 and L2. As a result, when the car crashes against the pole, the timing for spreading the air bag is delayed and the occupant cannot be protected appropriately.
The inventors have studied the correlation among the respective accelerations which are generated at a plurality of points of the car when the car crashes and the relationship between the correlation and crash modes of the vehicle.
The crash modes of the car (hereinafter referred to as a car A) include, mainly, a head-on crash, an oblique crash, an offset crash, an under-ride crash and a pole crash (see FIG. 10).
The head-on crash is a crash mode wherein the car A crashes against the front part of a vehicle W running in the opposite direction by the whole front end part thereof. In the head-on crash, the both front right and left frames of a frame Aa at the front end part of the car A crash against the front part of the vehicle W running in the opposite direction. It is noted that the frame Aa is normally formed of a highly rigid metallic material.
The offset crash is a crash mode wherein the car A crashes against the front right side of the vehicle W running in the opposite direction by the front right side thereof, for example. In the offset crash, the frame Aa of the car A crashes against the front right side of the vehicle W running in the opposite direction by only the right frame among the both front right and left frames.
The pole crash is a crash mode wherein the car A crashes against a pole P by the front center part thereof. In the pole crash, the pole P cuts into a part between the front right and left frames of the frame Aa. It is noted that the rigidity of the part between the front right and left frames among the frame Aa is very low as compared to that of the frame Aa.
It was found that respective components such as an engine located between the front right and left frames of the frame Aa hit to each other and show complicated behavior during the process of the pole crash when their behavior are studied.
The under-ride crash is a crash mode wherein the car A enters under the rear lower part of a preceding vehicle W by the front end part thereof as shown in FIG. 10.
The oblique crash is a crash mode wherein the car A crashes against the front part of the vehicle W running in the opposite direction obliquely by the front end part thereof.
Three acceleration sensors were used to study accelerations at a plurality of points of the car. One acceleration sensor (hereinafter referred to as an acceleration sensor X) was disposed at the center both in the longitudinal and in the right and left directions of the car A and the two remaining acceleration sensors (hereinafter referred to as acceleration sensors Y) were disposed at the positions closer to the front part than the acceleration sensor X on the both right and left sides of the car A.
The following result was obtained by studying the correlation among the integrated values of the detected accelerations, i.e., the velocity changes, of the above-mentioned respective acceleration sensors when the car A crashes.
That is, when the crash mode of the car A is the head-on crash, the velocity changes, i.e., the integrated values of the respective detected accelerations, of the two acceleration sensors Y are almost the same. Further, the respective velocity changes are greater than the velocity change of the acceleration sensor X when the head-on crash has occurred.
When the crash mode of the car A is either the offset crash or the oblique crash, for example, when the car A has caused the offset crash or the oblique crash by the right side thereof, the velocity change of the acceleration detected by the right acceleration sensor among the two acceleration sensors Y is greater than the velocity change of the acceleration detected by the left acceleration sensor. Still more, the velocity change of the acceleration detected by the left acceleration sensor is almost equal with the velocity change of the acceleration detected by the acceleration sensor X.
When the crash mode of the car A is either the pole crash or the under-ride crash, the velocity changes of the respective accelerations detected by the two acceleration sensors Y both change in complex while taking values close to the velocity change of the acceleration detected by the acceleration sensor X. It is considered to be caused by the complex behavior of the components such as the engine as described above when the crash mode is either the pole crash or the under-ride crash.